I'm trying to call a C function that expects a C string (char*) from go. I know about the C.CString function documented in the cgo documentation but as the function I'm calling will already make a copy, I'm trying to avoid the one Cstring makes.
Right now, I'm doing this, s being a go string
var cs *C.char = (*C.char)( unsafe.Pointer(& []byte(s) [0]))
But I get the feeling that the []bytes(s) is making its own copy. Is it possible to just get the char* ?
If you're doing this enough times that performance is a concern, it would really be advisable to keep the data in a slice to begin with.
If you really want to access to the address of the string, you can use the unsafe package to convert it into a struct matching the string header. Using the reflect.StringHeader type:
p := unsafe.Pointer((*(*reflect.StringHeader)(unsafe.Pointer(&s))).Data)
Or using a slice as a proxy, since they both put the data pointer and length integers in the same field locations
p := unsafe.Pointer(&(*(*[]byte)(unsafe.Pointer(&s)))[0])
Or because the data pointer is first, you could use a uintptr alone
p := unsafe.Pointer(*(*uintptr)(unsafe.Pointer(&s)))
https://play.golang.org/p/ps1Py7Ax6QK
None of these ways are guaranteed to work in all cases, or in future versions of Go, and none of the options are going to guarantee a null terminated string.
The best, supported option is to create a shim in the cgo preamble to accept the go string, and convert it to a *char. CGO provides access to the following function to do this:
const char *_GoStringPtr(_GoString_ s);
See the Go references to C section in the documentation.